Low temperature reflow method for filling high aspect ratio contacts

ABSTRACT

Impurities are added to a conductor layer in a semiconductor process to prevent formation of void spaces and encourage complete filling of contacts. The impurities reduce the melting point and surface tension of a conductor layer, thereby improving filling characteristics during a reflow step. The impurities may be added at any time during the process, including during conductor deposition and/or reflow.

This application is a Continuation of application Ser. No. 08/631,445filed Apr. 12, 1996 now U.S. Pat. No. 5,789,317.

The U.S. Government has a paid-up license in this invention and therights in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.MDA972-92-C-0054 awarded by the Advanced Research Projects Agency(ARPA).

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates generally to semiconductor devices, and morespecifically to conductor deposition processes for semiconductors. Inparticular, this invention relates to reflow processes used to fillhigh-aspect ratio contacts (or vias).

2. Description Of The Related Art

Due to increases in semiconductor packing densities, contact diametershave been reduced, thereby increasing height-to-width or aspect ratios.With increasing aspect ratios, adequate metal step coverage of contactsurfaces has become more difficult to achieve, especially attemperatures lower than about 200° C. As the aspect ratio increases,metal deposited at colder temperatures fails to produce good stepcoverage due to “necking” (or “cusping”) at the top corners of contacts.Necking is detrimental because it gives rise to voids at the bottom ofthe contact, leading to reliability and yield problems. In an attempt toaddress this problem, various techniques have been developed andemployed, including chemical vapor deposition of metals (CVD), laserreflow and aluminum reflow methods.

Of the methods used to fill contacts, the aluminum reflow method offersthe advantages of lower cost and fewer process steps. However, mostaluminum reflow processes proposed for filling high-aspect ratiocontacts have not gained widespread acceptance due to the need forelevated reflow temperatures which are less desirable, particularly forsecond and higher metal levels. In these aluminum reflow processes, thetemperature at which the metal layer is deposited is typically in therange of about 400° C. to about 500° C. These higher depositiontemperatures may cause voiding and discontinuities in the metal layerdue to the localized absence of sufficient metal to support continuousgrain growth. As the aspect ratio increases, this problem may worsenbecause the amount of metal deposited onto the contact bottom andsidewalls decreases due to necking. High reflow temperatures may furtherincrease the potential for voids by causing the early formation ofwidely spaced grains that lead to the formation of voids.

In an attempt to modify the aluminum reflow method for high aspect ratiouse with lower reflow temperatures, aluminum alloy materials have beenemployed to reduce the melting point of the metal layer. For example, anAl—Ge—Cu damascene process using low temperature reflow sputtering hasbeen used. In this process, Al—Ge—Cu alloy is deposited at roomtemperature onto the surface of contacts coated with Ti. The Al—Ge—Cu isthen annealed at a reflow temperature of about 400° C. The depositionand annealing steps are repeated as necessary to create a multi-levelmetallized interconnection. Although this method may be successfullyused to create multi-level interconnections at lower reflowtemperatures, it suffers from several problems. These problems mayinclude difficulty in etching Al—Ge—Cu due to precipitation of Ge, anddegradation in resistivity performance.

Although aluminum reflow processes are preferred due to low cost andsimplicity, other techniques have been developed for filling high aspectratio contacts. For example, flared or tiered contacts have been used toreduce the potential for necking at the contact corners. However,altering the geometry of contact corners results in the loss ofsemiconductor area. Low metal deposition rates have also been used toensure adequate coverage, however, low deposition rates increase cost bylimiting throughput.

Another method developed for filling high aspect ratio contacts utilizesa multi-step deposition process. In this process, a thin layer of metalis deposited at a cold temperature followed by the deposition of a thicklayer of metal at a temperature of about 400° C. to 500° C. However,this process does not entirely eliminate the production of void spaces.

In a laser reflow process, a metal layer is deposited and thenplanarized by reflowing the metal with a laser. However, this processdoes not work well with aluminum or aluminum based alloys. When used toreflow aluminum, a native oxide forms on the aluminum and preventsplanarization. Aluminum also requires a high optical pulse energy andvariations in its surface topography can increase absorbed power andresult in damage.

In another method, a metal layer is deposited in two steps using apartially ionized beam (PIB). In this method, a contact is first filledby a metal layer deposited at a temperature of about 150° C. After thecontact is filled, a second metal layer is deposited at a temperature ofabout 300° C. This process produces adequate results, however, it is notpractical for use in manufacturing applications due to a low depositionrate and a high substrate bias. The low deposition rate reducesthroughput and increases the risk of gaseous inclusion into the metallayer. The high substrate bias is hard to maintain at a constant leveland may damage semiconductor devices.

In yet another multi-step metallization process, a thick metal layer isdeposited on a semiconductor wafer at a cold temperature of about 200°C. or less. In a second step, the temperature is increased toapproximately 400° C. to 500° C. while additional metal is beingdeposited. Although this method reduces the tendency for void formation,voids may still be formed if insufficient metal is deposited prior toincreasing the temperature.

Still other techniques to improve the filling characteristics ofaluminum have been tried. These methods include altering the surfacecharacteristics of a contact by coating the contact with a material suchas titanium or TiN to improve the wettability and coating conformance ofaluminum. However, these methods suffer from reactions between aluminumand titanium that interfere with the reflow process, or require specialtreatment of a TiN layer, e.g. such as with a plasma treatment.

Consequently, a need exists for a practical and efficient method forfilling high aspect ratio contacts. In particular, a need exists for alow cost reflow process that may be used to fill high aspect contacts ina void-free manner.

SUMMARY OF THE INVENTION

The present invention in a broad aspect concerns a conductor depositionprocess for semiconductor devices in which reflow of the conductor isimproved by reducing the melting point and surface tension of theconductor. The invention also relates to a reflow process forefficiently filling high aspect ratio contacts at lower temperatures.

This invention, in one respect is a method of processing a semiconductorsubstrate having a surface with a contact formed therein, including thesteps of depositing a conductor layer on the semiconductor substratesurface, forming an impurity layer in the conductor layer having areduced melting point temperature and surface tension, and heating theconductor layer to a reflow temperature sufficient to cause the layersto reflow.

In another respect, this invention is a process for semiconductormetallization, including the steps of depositing a metal layer on asemiconductor wafer surface, exposing the exterior surface of the metallayer to a sufficient amount of an impurity to form an impurity layerhaving a reduced melting point temperature and surface tension, andheating the metal layer to a reflow temperature sufficient to cause thelayers to reflow.

In a further respect, this invention is a semiconductor device includinga semiconductor substrate having a first layer with a contact formedtherein, a conductor layer formed to extend into the contact, and animpurity layer in the conductor layer having a reduced melting pointtemperature and surface tension.

Using the method of the present invention, the reflow of a conductor isimproved by reducing the melting point and surface tension of theconductor by the addition of impurities. The resulting reduction in themelting point of the conductor makes it possible to efficiently fillhigh aspect ratio contacts at lower temperatures. By “efficient filling”it is meant that good coverage is achieved during the reflow processwith the creation of substantially no void spaces within the contact. Inthose embodiments of this invention where impurities modify the surfaceand not the bulk of the film, the etchability and electromigrationperformance of the conductor are generally not substantially degraded.

The present invention offers advantages over methods previously employedto fill high aspect contacts, including for example, low cost and simpleprocessing, efficient filling of contacts at lower temperatures,improved reliability, higher yield and little or no degradation in theresistance of underlying conductor layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a-c) are a sequential, cross sectional representation of thecreation of a void space in a contact during filling using prior artmethods.

FIGS. 2(a-c) are a sequential, cross sectional representation of anefficient void-free filling of a contact using an embodiment of thepresent invention having an impurity that migrates to the surface of anupper conductor.

FIGS. 3(a-c) are a sequential, cross sectional representation of anefficient void-free filling of a contact using an embodiment of thepresent invention having an impurity layer that does not migrate to thesurface of an upper conductor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, “contact” refers to a hole formed in a layer which is ona semiconductor wafer. The term “contact” may be used interchangeablywith the terms “contact hole” and “via”. The term “substrate” refers toany semiconductor substrate, such as, for example, a semiconductor wafersubstrate. It will be understood by those skilled in the art that theterm “substrate” may include either a semiconductor wafer or the waferalong with various process layers formed on the wafer. The term“impurity” may be used interchangeably with the term “impurities” andrefers to any material or combination of materials capable of loweringsurface tension and/or melting point of a conductor layer. The term“metals” is defined to include metals, refractory metals,intermetallics, or combinations thereof.

Conductor reflow filling occurs due to surface tension drivenmass-transport in a surface layer whose thickness depends upon thereflow temperature. In the present invention, the addition of animpurity reduces the melting point of a conductor, thereby lowering thesurface tension for flow into a contact. The mass transport of conductormaterial into a contact occurs due to surface stress vectors which aregenerally proportional to the product of the surface tension and surfacecurvature. Mass transport occurs much faster when the surface tension islower, resulting in quicker filling of a contact, thereby preventingnecking and void formation. In a preferred embodiment the conductor maybe a metal layer.

FIGS. 1(a-c) sequentially illustrate the filling of a high aspect ratiocontact using a hot sputtering process of the prior art which results inthe formation of a void space in the filled contact. In FIG. 1a, asemiconductor wafer 1 having a substrate, conductor or metal layer 2 anda dielectric or insulator layer 4 is represented. It will be recognizedby those skilled in the art that metal layer 2 may alternatively be someother conductor layer such as, for example, polysilicon, or may in factbe a semiconductor substrate, such as silicon. Dielectric layer 4 hasbeen etched to form a contact 8 for filling. During the process,conductor layer 6 is deposited on the semiconductor surface in such away that it coats the sidewall and bottom of contact 8. In FIG. 1b. heatis applied to semiconductor wafer 1 at the same time conductor layer 6is being deposited. As heat is applied. conductor layer 6 begins toreflow and forms a “neck” 10 at the top corners of contact 8.

As the reflow process proceeds in FIG. 1c, this neck eventually closesto seal the top of the contact 8, leaving a void space 12 within thecontact. The existence of the void space 12 is undesirable because itmay render the semiconductor device inoperable or cause reliabilityproblems.

FIGS. 2(a-c) sequentially illustrate the filling of a high aspect ratiocontact using a hot sputtering embodiment of the process of the presentinvention in which conductor deposition and reflow occur at the sametime. In FIG. 2a, a semiconductor wafer 13 having a substrate, conductoror metal layer 14, a dielectric or insulator layer 16, and a conductoror metal layer 18 deposited thereon is illustrated. As used herein,layer 14 will be referred to as lower conductor layer 14, and layer 18will be referred to as upper conductor layer 18. Those skilled in theart will recognize that lower conductor layer 14 may be formed from anynumber of metals or conductors, such as, for example, silicon,polysilicon, tungsten, titanium, aluminum, etc. Furthermore, upperconductor layer 18 may be formed from any number of metals orconductors, including aluminum, aluminum based metals (such as aluminumalloys), tungsten, titanium, copper, and alloys and combinationsthereof, etc. Most preferably, upper conductor layer 18 is aluminum oran aluminum alloy metal.

The exposed portion of the lower conductor layer 14 and the dielectriclayer 16 together form the semiconductor wafer surface 19. Prior todeposition of upper conductor layer 18, a contact 20 has been etchedinto the surface of the semiconductor wafer 19. Once deposition begins,upper conductor layer 18 has an interior surface 18 a in contact withthe semiconductor wafer surface 19 and an exterior surface 18 b not incontact with the surface of the semiconductor wafer. Although FIGS.2(a-c) illustrate the filling of a contact using a hot sputteringprocess, in the practice of this invention an upper conductor layer maybe deposited on a semiconductor substrate using any suitable conductordeposition technique known to the art, including evaporation,sputtering, hot sputtering, CVD and plasma CVD methods.

In FIG. 2b, impurities have been added during conductor deposition andreflow to form impurity layer 21. Thus formed, impurity layer 21 is aportion of upper conductor layer 18 that contains a desiredconcentration of impurities mixed with conductor. Although in FIG. 2bimpurities have been added during latter stages of deposition,impurities may alternatively be added at any time during a contactfilling process, including prior to deposition, after deposition,continuously and throughout deposition, intermittently during depositionand after deposition during a separate reflow step. In the practice ofthis invention, impurities may be migrating or non-migrating.Non-migrating impurities tend to remain essentially in place, before andafter reflow, with co-deposited conductor material. Migrating impuritiesmay migrate in many ways, including upward, downward, toward exterior orinterior surfaces of a conductor layer, or they may disperse throughouta conductor. Migration may be partial or substantially complete, and mayoccur during reflow and/or during other stages of a process.

In FIG. 2b, impurity layer 21 is formed by exposing upper conductorlayer 18 to impurities. Sources of impurities may be gaseous, liquid,solid, or mixed phase in nature, and may be exposed to a conductor atany point in a process using any suitable exposure or deposition methodknown to the art. For example, when added to a plasma or ambient duringhot sputtering or reflow, impurities may be added in a gaseous form tothe sputter chamber. Alternatively, a conductor may be deposited on asemiconductor substrate, followed by deposition of a solid impurity ontop of the conductor layer. This may be followed by deposition ofanother conductor layer, a reflow step or a combination thereof. Solidimpurities may be deposited using any suitable deposition technique.including evaporation, sputtering, hot sputtering, CVD and plasma CVD.When added between conductor deposition steps, solid impurities may bedeposited in the same or different chamber where conductor depositiontakes place. Once deposited, a solid impurity may be selected so as tomigrate into the surface of a conductor layer to form an impurity layerhaving a reduced melting point and/or surface tension. In addition tothe examples given above, solid impurity sources may also be exposed toa conductor during conductor deposition and/or reflow and gaseoussources may also be exposed to a conductor between or after conductordeposition and/or reflow steps. Combinations of gaseous, liquid andsolid impurity exposure steps are also possible.

In FIG. 2b, impurities in layer 21 reduce the conductor melting point,thereby lowering the surface tension of impurity layer 21. Becausefilling of the contact 20 during reflow occurs due to surfacetension-driven mass transport, reduced surface tension causes filling tooccur much faster and prevents necking at top corners 22 of contact 20.Because little or no necking occurs, good coverage of contact 20 isachieved and few, if any voids are formed, resulting in a solid metalcontact 24 as shown in FIG. 2c.

In FIG. 2c, impurity layer 21 is shown at exterior surface 18 b of layer18. In this embodiment of the present invention, impurities depositedduring latter stages of conductor deposition and reflow havesubstantially migrated toward exterior surface 18 b to form an impuritylayer 21 of generally uniform thickness essentially parallel to exteriorsurface 18 b. This migration leaves the remainder of conductor layer 18.including filled contact 24 substantially free of impurities. Suchupward migration may be desirable, for example, when impurities possessor impart unfavorable characteristics to conductor layer 18, such asdifficulty in etching or degradation in resistivity performance.Depending on the nature of a conductor layer, this may be the case forimpurities that include materials such as germanium or silicon.

FIGS. 3(a-c) illustrate an embodiment of the present invention in whichimpurities deposited during conductor deposition and reflow do notmigrate through conductor layer 18. In FIGS. 3(a-c), impurity layer 26has been formed using a hot sputtering deposition and reflow processsimilar to that used in FIGS. 2(a-c). As before. impurity layer 26 alsoexists at exterior surface 18 b. However, in this embodiment. impuritieshave not substantially migrated and impurity layer 26 extends downwardto form an impurity core or “plug” in contact 28. Such an impurity coremay be desirable when impurities possess or impart favorablecharacteristics to a conductor layer, such as electromigrationperformance. Depending on the nature of a conductor layer, this may bethe case for impurities containing materials such as copper. Althoughrepresentative embodiments of the present invention have beenillustrated in FIGS. 2(a-c) and 3(a-c), it is not intended that thepractice of the present invention be limited to those steps specificallyrepresented in FIGS. 2(a-c) and 3(a-c), nor that perfect void freefilling or complete coverage of a contact be required.

Although not illustrated, benefits of the present invention may also beobtained by depositing single or multiple impurity layers within aconductor layer below the surface. An impurity layer may be formedbeneath the surface of a conductor layer in many ways. For example,impurities may be “sandwiched” within a conductor layer by a depositionof essentially non-migrating impurities between two depositions ofimpurity-free conductor material. In this case, impurities may beintermittently deposited as part of one continuous conductor depositionand reflow process, or may be deposited in separate distinct stepsbetween deposition and/or reflow of conductor material and previousimpurity layers. This process may be repeated any number of times Withnon-migrating impurities to form stratified layers of impurities ifdesired. Migrating impurities may also be deposited in this manner ifdesired, although stratified impurity layers may not be formed.

In the practice of the present invention, impurities that may be addedto reduce the melting point and surface tension of a conductor depend onthe type of conductor selected. Impurities may be added to form animpurity layer taking up any desired portion of the overall thickness ofa conductor layer, including up to 100% of a conductor layer, butpreferably from about 20% to about 80%. So done, the melting point andsurface tension of a conductor will be reduced in the region of aconductor layer corresponding to the thickness of such an impuritylayer.

Desired reduction in melting point and/or surface tension depend on thetype of conductor material present in a conductor layer. Preferably,sufficient impurities are added to reduce the melting point of aconductor in an impurity layer by from about 10% to about 60% of theintrinsic melting point of a conductor. By “intrinsic melting point” itis meant the melting point of a pure element that forms the predominantcomponent of a conductor. Amount of impurities added depends on theconductor selected, the reduction in melting point desired, thethickness of impurity layer desired, and the step in the filling processwhere impurities are added.

In one preferred embodiment of the present invention, a conductor layeris comprised of aluminum based metal which is deposited on asemiconductor wafer surface using a hot sputtering process. In thisembodiment, an aluminum based metal layer is preferably deposited andsimultaneously reflowed at as low a temperature as possible. For purealuminum, reflow temperature is typically greater than about 300° C. andtypically less than about 550° C., preferably below about 500° C., andmost preferably below about 400° C. In this embodiment, impurities areadded by gas flow during about the last 30% of conductor deposition, anddeposition and reflow temperature is controlled by amount and type ofimpurities selected. Acceptable impurities for reducing melting pointand surface tension of aluminum based metals, such as pure aluminum,include any suitable gaseous impurity sources known to the art capableof lowering surface tension and melting point of a conductor. Suchsources include, but are not limited to. gaseous impurity sourcescontaining silicon, germanium, one or more halogens and mixturesthereof. Some examples of such suitable impurity sources include silane,disilane, germane, GeF₄, SiF₄, Cl₂F₂, ClF₃, ICl₃, ICl₅, TiCl₄. In thisembodiment. TiCl₄ is preferred. Although gaseous impurity sources areused in this embodiment, any suitable solid impurity source known to theart capable of lowering melting point and/or surface tension of aluminumbased conductor layers may also be used successfully in the practice ofthis invention. Such solid sources include but are not limited to metalsand metal based materials, such as WF₆ and TaCl₅.

The reflow process disclosed herein may be utilized in the formation ofa wide range of semiconductor devices, including both MOS devices andbipolar devices. For example, the reflow process may be used for forminglogic devices, microprocessors or memory devices, such as DRAMs, SRAMsor ROMs.

Although some preferred embodiments of the method of the presentinvention are practiced using a one-step conductor deposition and reflowprocess, the method of the present invention may also be employed usinga multi-step deposition process. When a multi-step deposition process isused, impurities may be added prior to, after, or during any or all ofthe deposition steps to improve the filling characteristics of aconductor. In addition, conductor reflow may occur simultaneously orseparately with any or all conductor deposition and/or impuritydeposition steps.

Although the invention has been described by reference to preferredembodiments, it is not intended that the novel methods and compositionsbe limited thereby, but various modifications are intended to beincluded as falling within the spirit and broad scope of the foregoingdisclosure and the following claims.

What is claimed is:
 1. A method of forming a contact, the methodcomprising the steps of: (a) providing a substrate having a contact holeformed therein, the contact hole exposing a portion of a conductive areaof the substrate; (b) depositing conductive material comprising aluminuminto the contact hole at a temperature that causes the conductivematerial to reflow, the conductive material having a surface tension anda melting point; and (c) depositing an impurity intermittently into theconductive material while the conductive material is being deposited,the impurity causing at least one of the surface tension and the meltingpoint of the conductive material to lower.
 2. The method, as set forthin claim 1, wherein the conductive material is deposited within atemperature range of about 300 degrees Celsius to about 500 degreesCelsius.
 3. The method, as set forth in claim 1, wherein said impurityis derived from an impurity source containing at least one of silicon,germanium, a halogen, a metal, and a metal-based material.
 4. Themethod, as set forth in claim 1, wherein step (c) comprises the step ofdepositing impurities which migrate out of the contact hole.
 5. Themethod, as set forth in claim 1, wherein step (c) comprises the step ofdepositing impurities which do not migrate out of the contact hole. 6.The method, as set forth in claim 1, wherein step (c) comprises the stepof lowering the melting point of the conductive material by 10% to 60%.7. The method, as set forth in claim 1, wherein the impurity isdeposited after 70% of the conductive material has been deposited.
 8. Amethod of forming a contact, the method comprising the steps of: (a)providing a substrate having a contact hole formed therein, the contacthole exposing a portion of a conductive area of the substrate; (b)depositing conductive material comprising aluminum into the contact holeat a temperature within a range of 300 degrees Celsius to 500 degreesCelsius, the conductive material having a surface tension and a meltingpoint; and (c) depositing a gaseous impurity comprising TiCl₄ into theconductive material after about 70% of the conductive material has beingdeposited, the impurity causing at least one of the surface tension andthe melting point of the conductive material to lower.
 9. The method, asset forth in claim 8, wherein step (b) comprises the step of depositingthe conductive material at a temperature below about 400 degreesCelsius.
 10. A method of filling a feature having a high aspect ratio,the method comprising the steps of: (a) depositing conductive materialinto the high aspect ratio feature at a temperature that causes theconductive material to reflow, the conductive material having a surfacetension and a melting point; and (b) depositing an impurityintermittently into the conductive material while the conductivematerial is being deposited, the impurity causing at least one of thesurface tension and the melting point of the conductive material tolower.
 11. The method, as set forth in claim 10, wherein the conductivematerial comprises aluminum and is deposited within a temperature rangeof about 300 degrees Celsius to about 500 degrees Celsius.
 12. Themethod, as set forth in claim 10, wherein said impurity is derived froman impurity source containing at least one of silicon, germanium, ahalogen, a metal, and a metal-based material.
 13. The method, as setforth in claim 10, wherein the conductive material comprises at leastone of aluminum, aluminum alloy, tungsten, tungsten alloy, titanium,titanium alloy, copper, and copper alloy.
 14. The method, as set forthin claim 10, wherein step (b) comprises the step of depositing animpurity which tends to remain in place with the conductive materialdeposited therewith.
 15. The method, as set forth in claim 10, whereinstep (b) comprises the step of depositing an impurity which tends tomigrate from a place relative to the conductive material depositedtherewith.
 16. The method, as set forth in claim 15, wherein step (b)comprises the step of depositing an impurity which migrates out of thehigh aspect ratio feature.
 17. The method, as set forth in claim 15,wherein step (b) comprises the step of depositing an impurity whichdisperses throughout the conductive material.
 18. The method, as setforth in claim 10, wherein step (b) comprises the step of lowering themelting point of the conductive material by 10% to 60%.
 19. The method,as set forth in claim 10, wherein the impurity is deposited after 70% ofthe conductive material has been deposited.